The invention relates to a coupling, either in the form of an conventional viscous coupling or in the form of a Viscotrac.RTM. coupling, comprising two parts which are supported inside one another, which are rotatable around a common longitudinal axis and which form an annular space which is filled with a highly viscous medium and in which there are arranged inner plates which are non-rotatably connected to the inner one of the parts and form a set of inner plates, as well as outer plates which are non-rotatably connected to the outer one of the parts and form a set of outer plates, said inner plates and outer plates being arranged so as to alternate in the longitudinal direction for the purpose of generating a coupling effect between the two parts when the latter rotate relative to one another.
In the case of conventional viscous couplings such as those described in DE 38 23 180 C1, the differential-speed-dependent or wheel-slip-dependent engaging effect is based on the build-up of shear forces in the viscous medium in the spaces between inner plates and outer plates.
In the applicant's Viscotrac.RTM. coupling described in DE 196 53 310 A1, differential-speed-dependent or wheel-slip-dependent shear forces are built up in the viscous medium in the spaces between inner plates and outer plates in the same way as described above. In addition, in a helical channel between a cylindrical surface of an annular piston dividing the annular space into two chambers and a counter face of the annular space, a medium is conveyed by shear forces from the one chamber into the other chamber, so that the annular piston is axially displaced in the annular space and moves part of the inner plates and outer plates of the coupling plate type into friction contact.
Couplings of said type are used in the driveline of a motor vehicle which comprises both a constantly driven axle and an axle that is driven only if there exists slip between the wheels of the axles, the couplings being provided in the connecting driveline between the two axles.
Under operational conditions, for example, when accelerating from start conditions or during off-highway driving or in curves, when at the constantly driven axle there is wheel-slip, there occurs a relative rotation between the coupling parts, as a result of which a coupling effect is built up, so that the second axle is also driven. In the case of vehicles with new types of driving dynamics systems which, for instance, cause the brake to engage in individual wheels if lateral guidance is lost at the wheels, just as in the case of vehicles with anti-lock braking systems which release the braking force when a wheel is locked, control must be affected only to a slight extent by said coupling engagement. It is therefore the object of the present invention to provide a coupling of the above-mentioned type which is more compatible with control systems for driving dynamics and with anti-lock braking systems of motor vehicles.